The '20's Plenty for Us' campaign suggests that traffic in cities flows more freely with a 20mph limit than when the limit is 30mph.

The campaign argues that while many people wrongly assume that 20mph limits delay journey times, in fact average city speeds are generally well below 20mph because of congestion and queues. The campaign for lower speed also claims that in 20mph zones drivers make better use of road space and junctions work more efficiently and at a higher capacity, as it is easier to merge.

20s Plenty for us says that "optimal speeds for maximum urban traffic flow have been mathematically modelled and 20mph is more efficient than 30mph". The campaign also claims that walking and cycling levels rose by up to 12% after 20mph limits were introduced in Bristol.